A comparator in use typically is arranged to compare a varying input voltage with a fixed reference voltage and to provide an output signal transition or trigger whenever an excursion of the input crosses the reference. In critical applications where inaccurate or slow trigger would adversely affect co-operating circuits, a high gain operational amplifier may be used in a feed back configuration to provide a fast and repeatable trigger. Unfortunately in order to provide noise immunity by hysteresis, high value resistors are required in the feed back path and consequently such an arrangement cannot be successfully integrated.
A comparator is essentially a differential device producing an output signal dependent upon the difference between two inputs. As such a differential stage is required at the input and a single-ended stage is required to provide the output. An example of a differential stage which may be successfully integrated is the classical long tail transistor pair in which two transistors with common emitters are supplied with a constant current, each transistor receiving one of the signals for comparison at its base. The configuration is symmetrical and varies between a first stable state (input below reference, for example) wherein the transistor on the input side sustains the entire current flow and a second stable state (input above reference), wherein the transistor on the reference side sustains the current. Between the two stable states (input approaching reference) the arrangement enters an active region while the transition from one state to the other occurs. In this region the current is shared by the transistor pair dependent upon the precise relationship between the input and the reference.
An output (either inverted or not depending upon the selection) maybe provided by a direct connection to the collector of a transistor of the pair. Effectively being high level or low level depending upon whether the transistor is conducting or not. During transition however such an output is input dependent and variations in the input will appear at the output. Such input tracking prevents a clean transition and is undesirable. Improvements can be made if the transition time can be shortened or if the output is provided by a further stage (a transistor arranged as a switch for example) optimized to toggle between fixed output values, with a reduced active region of its own. If complementary inverting and non-inverting outputs are required a further output device toggling in the opposite sense may be used.
To shorten the transition time a regenerative arrangement may be used in which feedback is applied to one of the transistor bases acting in the same sense as the transition, thus the transition is regenerative and no longer relies on the input to attain the stable state. However, the feedback cannot be applied until the transition has begun to a substantial extent and even with such arrangements the configuration passes through a stage of non-regenerative transition.
Arrangements employing both regenerative differential techniques and optimised output stages have been successfully integrated, however their performance remains inferior to that of the feedback operational amplifier and may be unacceptable in some critical applications.
Consider, for example, a microprocessor controlling critical plant. If the power supply to the processor were to fail momentarily and then be restored, the processor may enter some random state and the control be invalid with potentially disastrous results. A solution is to monitor the supply voltage and if it falls below a certain level to provide a reset signal to the processor. A comparator may be used for this purpose of monitoring the supply level, and it is desirable that an integrated comparator be used so that all supply management functions required by the processor may be integrated within a single device.
Unfortunately comparators which have been integrated to date have proved inadequate for this purpose due to problems of glitching (spurious pulses without full trigger) and, in complementary output types, output overlap, particularly in applications where the input is subject to slow variation.